Why put wall on 4th column? [closed]

It's likely additional open space is used to maintain more open space to use plants such as the Cherry bomb effectively.

You can also argue that, the effective firepower you need to survive not only depends on the amount of zombies, but also how far they have to walk until they reach your first plant. If you have more open spaces, the zombies need to walk further, and you can kill more before they start eating. Of course, more open spaces means less plants, so there's a balancing act. Say it takes Bucket zombie 5 ticks to walk a tile, and 20 hits to kill him, with peashooter doing one hit per tick. With 8 peashooters and 1 tile, they can dish out 40 damage; 3 zombies will thus overwhelm these plants. But, with 7 peashooters and 2 tiles of space, they can dish out up to 70 damage: 3 zombies will not reach the front most peashooter: 4 are needed. The tendency of zombies to attack in large waves makes this tactically expedient.